(a) Field of the Invention
This invention relates to coated aspirin tablets or caplets stabilized against degradation of acetylsalicylic acid. More particularly, this invention relates to coated aspirin tablets or caplets wherein decomposition into acetic acid and salicylic acid is inhibited by incorporation therein prior to coating of a small amount of citric acid, alginic acid, glutamic acid or admixtures of any two or three thereof.
(b) Prior Art
Alginic acid is disclosed in admixture with aspirin, phenacetin, caffeine starch and talc and further with these ingredients and polyvinylpyrollidone in an article by M.R. Nazareth et al entitled "Stability of Aspirin in Aspirin, Phenacetin, and Caffeine Tablets" in Journal of Pharmaceutical Sciences, Vol. 50, No. 7, July 1961. The article concludes with the statement: "Taking appearance, disintegration time, and stability of aspirin into consideration, formula B [without alginic acid] emerges as the best of the four formulations studies and can easily be used in preparation of tablets of aspirin, phenacetin, and caffein."
The "United States Pharmacopeia" 21st rev. Mack Publishing Company, Easton Pa. 1985 at page 79 specifies that aspirin tablets contain not less than 95.0% and not more than 105.0% of the labelled amount of acetylsalicylic acid (C.sub.9 H.sub.8 O.sub.4). Aspirin tablets are known to decompose slowly into acetic acid and salicylic acid. The assay at the said USP page 79 for Free Salicyclic Acid indicates that for tablets not more than 0.3% is found and for coated tablets not more than 3.0% is found. The rate of decomposition is slow but in order for the tablets to meet their labelled shelf-life according to USP standards, the shelf life is sometimes shortened to be within safe limited. Recently it has been found that with the advent in the market place of film coated tablets, such as those described in U.S. Pat. No. 4,302,440, the decomposition of such tablets appears to be accelerated by the coating in that the acetic acid formed by the decomposition is trapped within the tablet and causes further decomposition. Also, the acetic acid seems to impart a vinegar taste to the coated tablets after standing. There is therefore a need in the art for an inhibitor of acetylsalicylic acid decomposition in order to extend the shelf life of aspirin tablets and caplets, either film coated or enteric coated, the latter as described in U.S. Pat. No. 4,017,647.